The present invention relates to a system for controlling a lock-up system of an automatic transmission for a motor vehicle.
In the automatic transmission having a lock-up system, when a torque converter provided in the transmission is locked up by the lock-up system, the torque of an engine may tend to change. FIG. 5a shows a torque curve of an engine showing the relationship between torque and engine speed. When the torque converter is locked up, the torque converter is prevented from slipping, so that engine load increases. As a result, engine speed slows down. For example, if the torque converter is locked up at an engine speed A in FIG. 5a, the engine speed slows down to a speed B by .DELTA.NE. On the other hand, engine torque increases from D to C by .DELTA.T. Further, the power of the engine also increases as shown in FIG. 5b. In FIG. 5b, the solid line S shows the power in the lock-up state with respect to vehicle speed, and the dotted line L shows the power in the torque converter operating state (non-lock-up state). When the torque converter is locked up at a vehicle speed (about 45 Km/h), the power increases by .DELTA.F. By such increases of engine torque and power, the vehicle is unnecessarily accelerated, which gives an unpleasant feeling to the driver.
In order to eliminate such a defect, Japanese Patent Laid Open No. 58-42861 provided a system which operates to delay of the locking up of the torque converter until the engine output becomes stable. However, such a system reduces the driveability of the vehicle.